In the recovery of hydrocarbons, such as oil and gas, from natural hydrocarbon reservoirs by means of drilled wells, extensive use is made of wellbore treatment fluids such as drilling fluids, completion fluids, work-over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like. Such fluids may be required to be more viscous than water. They may be required to develop viscosity over time or after a delay. For some applications, notably hydraulic fracturing, it is desirable to reduce the viscosity after the fluid has been placed below ground, so as to assist its removal after it has served its purpose.
It is commonplace to use water-based fluids, rendered viscous by the incorporation of a thickening agent, usually based on a polymer. Guar, which is a polysaccharide, is frequently used. Other polysaccharides have also been used and guar or other polysaccharides may be used in a chemically modified form. When a viscosifying polysaccharide is used alone, it enhances viscosity. When cross-linked, which is normally done with borate or metal ions, viscosity is increased considerably further. Cross-linking may occur after the fluid has been pumped down the wellbore: this is advantageous in that the viscosity of the fluid during pumping and during travel down at least part of the wellbore is less than the viscosity exhibited within the fluid reaches its underground destination.
Hydraulic fracturing involves breaking or fracturing a portion of the formation around the wellbore by injecting fluid at pressures sufficient to initiate and extend a fracture in the formation through which hydrocarbon can more easily flow to the wellbore. The viscosified fluid is also used to transport a particulate proppant into the fracture to keep the fracture from closing completely once the pumping operation is completed. It is desirable that the fluid loses its viscosity after fracture formation so that it can more easily flow back from the wellbore when gas or oil production starts or resumes. It is also desirable that the filtercake of thickening polymer which may form at the boundary of the fracture can be removed when no longer required. For these reasons it is normal to include a delayed-action polymer-degrading material in a fracturing fluid thickened with polymer.
As an alternative to polysaccharides as thickeners, WO 2002/070861 teaches that the polymer in a fracturing fluid may be a synthetic block copolymer. WO 2006/075154 also uses block copolymers with an objective that the viscosifying polymer will degrade into soluble fragments when no longer required. This document suggests that some of the blocks may be chemically modified polysaccharide: US2009/0126932 proposes to use the Diels-Alder reaction to couple a polymer to a cross-linking agent and/or use the reverse Diels-Alder reaction to uncouple a polymer from a cross-linking agent and thereby regulate the viscosifying effect of the polymer.